1114 



THE LIGHT FACTOR. II. QUANTUM YIELD 



CHAP. 29 



few exceptionally high figures) from 6.9 to 12.9; the average for non- 

 alkaline media (methods 1, 3, and 4) was I/7 = 7.85; for alkaline buffers 

 (method 2) about 10. Kok estimated I/7 = 6.75 as the most probable 

 lowest value of the quantum requirement. 



The most interesting (and controversial) finding of Kok was that linear 

 extrapolation of the light curves to 7 = consistently lead to considerably 

 smaller values of the gas exchange than would have corresponded to the 

 respiration of the same cells in the dark. Upon closer study, he concluded 

 that the light curve underwent a sudden change of slope by a factor of 

 about 2, somewhere near the compensation point (fig. 29. 4B). He took 

 this to mean that the quantum efficiency was constant from near the 

 saturation point down to the compensation region, and then doubled sud- 

 denly. This shape of the light curve — according to Kok it consists of three 

 practically linear segments — has not been found by any of the previous 

 observers; however, Kok claimed a confirmation of the sharp break in the 

 P = /(/) curve by new analysis of the data of Kopp and of Gabrielsen. 



If the slope of the light curve changes by a factor of two at the com- 

 pensation point, the rate of respiration in strong light, determined by linear 

 extrapolation of the light curve from above the compensation point to 

 7 = 0, must indicate a rate of respiration in light equal to one half of the 

 rate of respiration in darkness. 



Later (1949) using a more precise manometric device (a "differential 

 volumeter") Kok found, as an average of 50 experiments with Chlorella 

 cells grown in Knop's medium, Rugu = 0.5 Rdnvk- (To increase /^dark, 



3 

 O 



O 



1 



o 



O 



£ 



200 



100 







-100 



/3 :>^ lU 15 



^^y I (relative units) 



B 



With glucose 



Fig. 29.4B. Light curves of photosynthesis at low liglit intensity after Kok (1949), 

 showing knick near compensation point. 



